Monitoring of an antenna system

ABSTRACT

The present invention relates to a method and apparatus for monitoring an antenna system comprising at least two antenna branches providing antenna diversity. At least a first signal branch measure and a second signal branch measure are repeatedly generated in response to a first signal branch and a second signal branch received by a respective one of the at least two antenna branches, thus generating a first plurality of second signal branch measure values in a manner so that the first and second pluralities reflect the quality of the first and second signal branches, respectively, at different points in time. The first and second pluralities are analysed in order to distinguish any differences in the performance of the first and second antenna branches.

FIELD OF THE INVENTION

The present invention relates to the field of radio communication, andin particular to the monitoring of antenna systems providing antennadiversity.

BACKGROUND

An antenna system is a most vital part in a mobile radio communicationssystem. Errors in the antenna system give rise to a reduced coveragearea and a decrease in the information throughput. Hence, it isimportant to monitor the state of the antenna system.

One method of monitoring the state of an antenna system is to performVoltage Standing Wave Ratio (VSWR) measurements, where a transmittedsignal and the reflected part of the transmitted signal are determinedand compared in order to determine the state of the antenna system.However, this method can only be used for the monitoring of antennasystems in radio base stations which comprise both a transmitter and areceiver.

Another method of monitoring antenna systems is to measure the DC feederresistance. However, since the feeder resistance varies as equipment,such as filters, amplifiers etc., are introduced to or removed from, theantenna system, this method requires knowledge of which equipment ispresently connected to the antenna system.

The above mentioned methods do not explicitly show whether the receiveris affected, but rather that the impedance of the antenna system haschanged. A further problem with the above mentioned methods is thatmanual configuration of the measurement equipment is generally required,since there exist many different types of antenna configurations andtransceiver/receiver units.

SUMMARY

A problem to which the present invention relates is how to improve theperformance of an antenna system.

This problem is addressed by a method of evaluating an antenna systemwhich are connected to a radio device, where the antenna systemcomprises at least two antenna branches. The method comprises receivinga radio signal as a first signal branch in a first antenna branch:receiving the radio signal as a second signal branch in a second antennabranch. The method further comprises repeatedly generating a firstsignal branch measure of the first signal branch and a second signalbranch measure of the second signal branch, thus generating a firstplurality of first signal branch measure values and a second pluralityof second signal branch measure values in a manner so that the first andsecond pluralities reflect the quality of the first and second signalbranches, respectively, at different points in lime; and analysing thefirst and second pluralities in order to distinguish any differences inthe operation of the first and second antenna branches.

The problem is further addresses by a radio device adapted to beconnected to at least a first and a second antenna branch on which afirst and a second signal branch, respectively, may be received. Theradio device is arranged to repeatedly generate a first signal branchmeasure of the first signal branch and a second signal branch measure ofthe second signal branch: thus generating a first plurality of firstsignal branch measure values and a second plurality of second signalbranch measure values in a manner so that the first and secondpluralities reflect the quality of the first and second signal branches,respectively, at different points in time. The radio device furthercomprises a signal branch comparator having an input arranged to receivesignals indicative of the first and second signal branch measures, andbeing adapted to analyse the first and second pluralities in order todistinguish any differences in the operation of the first and secondantenna branches.

By the inventive method and radio device is achieved that anydifferences in performance of the antenna branches may be detected. Suchdifferences give an indication that an antenna branch may experience afault. By ensuring that faults in the antenna system can be detected andattended to, the performance of the radio device will improve, and theoverall performance of a radio system of which the radio device forms apart will be improved.

The invention can advantageously be applied to a radio device operatingaccording to code division multiple access, where at least onescrambling code and/or channelisation code are applied to the first andsecond signal branches in order to de-scramble and/or de-spread thefirst and second signal branches, respectively. In this context, thefirst and second signal branch measures can advantageously be generatedin response to the de-scrambled and/or de-spread first signal branch andthe de-scrambled and/or de-spread second signal branch, respectively.The de-scrambled/de-spread signal branches show a highersignal-to-interference-and-noise-ratio than the signal branches beforede-scrambling/de-spreading, and a difference in the quality of thesignal branches can therefore more easily be discerned afterde-scrambling/de-spreading than before.

In case of the radio device comprising a rake receiver, the first andsecond branch measures can advantageously be obtained in response to anoutput from the rake receiver. In one embodiment of this aspect of theinvention, samples are obtained of the first and second signal branches,thus forming a first and second stream of samples obtained from thefirst signal branch and the second signal branch, respectively. The rakereceiver has a set of rake fingers operative to select a set of samplesfrom the first and second streams of samples: and the first signalbranch measure is obtained in response to the number of samples selectedby the rake receiver from the first stream of samples during a timeperiod; and the second signal branch is obtained in response to thenumber of samples selected by the rake receiver from the second streamof samples during the time period.

The first and second signal branch measures may alternatively beobtained as signal-to-noise-and-interference-ratio values obtained onthe first and second signal branches, respectively. When the radiodevice comprises a rake receiver, thesignal-to-noise-and-interference-ratio values of an antenna branch mayadvantageously be obtained from signals processed by the rake receiver.

The signal branch measure of a signal branch may be obtained fromsignals received on any logical channel, such as a random access channelor a traffic channel. The random access channels are often processed onthe same circuit board for all antenna branches, and the process ofidentifying any erroneous antenna branch could easily be centralised ifthe signal branch measures are performed on the random access channelsignals, thus rendering the construction of the signal branch comparatorsimple.

The comparison of the signal branch measures can be performed on aper-measurement basis, or performed on an average signal branch measurecalculated as the average value of a plurality of signal branch valuesobtained on a signal branch. By performing a per-measurement comparison,the differences in signal-to-noise-and-interference ratio values betweensignals originating from different radio transmitters experiencingdifferent radio conditions can be compensated for.

The problem is further addressed by a computer program productcomprising computer program code operable to execute the inventivemethod when run on computer means.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic drawing of a radio base station employing antennadiversity.

FIG. 2 is a flowchart illustrating an embodiment of the invention.

FIG. 3 is a schematic drawing of a radio base station comprising a rakereceiver.

FIG. 4 is a flowchart illustrating an embodiment of the invention.

FIG. 5 is a distribution graph illustration a method of analysing data.

FIG. 6 is a schematic drawing of a radio base station in which anembodiment of the invention is incorporated.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of a radio base station 100comprising an antenna system 105, and a signal processing part 110. Inthe signal processing part 110, radio and base band processing of asignal received by the antenna system 105 is performed. The processingcould for example include demodulation of the signal so that a digitalversion of the signal is obtained, de-coding, de-interleaving,de-multiplexing and rake receiving. In a mobile radio system operatingaccording to spread spectrum multiple access/code division multipleaccess (CDMA), the processing performed in the signal processing part110 of the radio base station 100 includes de-scrambling of the signalby applying the appropriate scrambling codes to the digital signal inorder to retrieve the signals transmitted by different transmitters(typically, different mobile stations). The signal processing part 110of a radio base station operating according to a spread spectrummultiple access/code division multiple access standard typically furtherapplies a channelisation code to the retrieved transmitted signal inorder to de-spread the signal.

The antenna system 105 is connected to the signal processing part 110via a feeder 115. The radio base station 100 would typically beconnected, via a transport part (not shown), to a base stationcontroller (BSC) or a radio network controller (RNC) of a mobile radiocommunications system, where the radio base station 100 would provideradio links for communication with mobile stations roaming in the mobileradio communications system.

Antenna system 105 can comprise antenna(s) for signal reception, and/orantenna(s) for signal transmission. Furthermore, in most mobile radiocommunications systems, antenna diversity is employed. An antennaemploying antenna diversity comprises two (or more) antenna elements sothat a signal can be received and/or transmitted by two (or more)different antenna elements at the same time. Such different antennaelements could e.g. comprise different physical antennas placed atdifferent points in space—so called space diversity—or different antennaelements for receiving signal elements of different polarity—so calledpolarisation diversity. By introducing antenna diversity, the negativeeffects of fading are reduced, since the fading conditions are not thesame at different points in space (space diversity) or for differentpolarisations of the signal (polarisation diversity). By receiving thesignal at two or more different antenna elements for which the fadingconditions are basically uncorrelated, the probability of receiving aversion of the signal having sufficient signal strength is greatlyincreased. The different antenna elements providing antenna diversity toan antenna is often referred to as different antenna branches. In FIG.1, the antenna system 105 comprises two antenna branches 120A and 120B,respectively connected to the signal processing part 110 via feeders115A and 115B. The antenna branches 120 could be realized by spacediversity, as in FIG. 1, by polarisation diversity, a combination of thetwo, or any other type of antenna diversity.

An antenna system 105 employing antenna diversity on the receivingsignals thus receives different versions of a received signal on thedifferent antenna branches 120. According to the invention, thedifferent versions of the received signals can be compared in order todetect any errors that have occurred on one of the antenna branches. Inthe following, the different versions of a signal that have beenreceived on different antenna branches 120 will be referred to asdifferent signal branches.

By acquiring comparison results from the different received signalbranches at several points in time, an indication can be obtained as towhether or not the different antenna branches 120 operate in the sameway. If one antenna branch 120 generally gives rise to a weaker signalbranch than the other(s), this is a good indication that the operationof this antenna branch 120 may be faulty. Advantageously, the signalbranches of many received signals are analysed in order to obtain goodstatistical certainty of the result.

According to the invention, a signal branch measure, indicating thequality of the signal received on a signal branch, is obtained atseveral points in time for the different antenna branches 120 that areto be compared. When comparing the signal branch measures for more thantwo different branches, the comparison could preferably be performed ona two-and-two basis.

An embodiment of the inventive antenna system evaluation process isschematically illustrated in FIG. 2. The antenna system evaluationprocess of FIG. 2 could be performed continuously, at regular intervals,or on demand.

A signal is received at different antenna branches 120. In step 205, avalue of a signal branch measure, indicative of the signal quality ofthe signal branch received by an antenna branch 120, is collected forthe antenna branches 120 that are to be compared. In step 207, it ischecked whether enough values of the signal branch measures have beencollected. This check could for example involve checking whether atimer, set before the first signal branch measure measurement was taken,has reached a pre-determined time, or whether a counter, which isincremented each time a signal branch measure measurement is taken, hasreached a pre-determined number. Such pre-determined lime period couldtypically be in the order of minutes (e.g. 2 minutes), and a typicalpre-determined number of samples could for example be in the order of1000. If enough samples have not been collected, then step 205 isre-entered and another signal branch measure is collected from each ofthe antenna branches 120 that are part of the comparison. In step 215,the values of the signal branch measures are compared for differentsignal branches, for example by calculating the ratio or the differencebetween the average values of the signal branch measures of the antennabranches 120, in order to determine whether the quality of the differentbranches differ significantly. Different signal branch measures, as wellas different methods of comparing the signal branch measures, arefurther discussed below.

In the embodiment of the invention illustrated by FIG. 2, the step ofcollecting of signal branch measure values of steps 205 and 207 iscompleted before any evaluation and comparison of the signal branchmeasure results take place in step 215. Needless to say, the comparisonof the collected signal branch measure values could start while newsignal branch measure values are still being collected.

When more than two antenna branches 120 are present, several comparisonsof the signal branch measure values could advantageously be made on atwo-and-two basis.

If it is found in step 215 that the signal quality of any of the antennabranches 120 differ significantly from the other branches, step 220 isentered wherein appropriate action is taken, such as e.g. the issuing ofan alarm or the start-up of further investigation processes. Step 230 isthen entered, in which an instruction to start the antenna systemevaluation process is awaited. If the comparison in step 220 does notindicate any erroneous performance of any of the antenna branches 120,step 230 is entered without first entering step 225.

Rather than measuring the signal branch measure of the first and secondantenna branches, the signal branch measure of the combined signal fromtwo antenna branches 120 could be measured/estimated in step 205 of FIG.2, as well as the signal branch measure of one of the signal branches305. The signal branch measure value of the second signal branch 305 forwhich a signal branch measure value is desired can then easily bederived as the difference between the signal branch measure value of thecombined signal and the signal branch measure value of the first signalbranch.

Branch information of the received signal is normally known within theradio part of the signal processing part 110, wherein demodulation ofthe received signal takes place, and the base band part, wherein furtherprocessing of the de-modulated signal takes place. The comparison canthus be made in the radio part of the signal processing part 110 as wellas in the base band part of the signal processing part 110. In a mobileradio communication system operating according to the Wideband CodeDivision Multiple Access (WCDMA) standard, the base band part of thesignal processing part 110 is generally implemented in the “Node Buplink user data processing for Uu-interface (User equipment-networklayer part 1)”.

When the comparison is made in the radio part of signal processing part110, a useful signal branch measure to be used in the comparison couldfor example be the received signal strength as measured in the radiopart (which can for example be measured as “RXLEV” in a system based onthe GSM standard, and as “RSCP” in a system based on the WCDMA standard.The received signal strength is particularly useful as a signal branchmeasure in the comparison between different signal branches insituations where the signal-to-noise-and-interference ratio iscomparatively high, so that the received signal strength effects ofinterference or noise on the received signal strength is low. This istypically the case in a mobile radio communication system based on theGSM standard.

In a system based on spread spectrum multiple access/code divisionmultiple access (CDMA), such as a mobile radio communications systembased on the WCDMA standard, the signal-to-noise-and-interference levelsare typically comparatively low, and it is often difficult to discernany difference in the signal-to-noise level of the different antennabranches 120. However, the present invention acknowledges that after thereceived signal has been de-scrambled by applying the appropriatescrambling code and/or has been de-spread by use of appropriatechannelisation codes, the signal-to-noise-and-interference-ratio isincreased. By performing the comparison between antenna branches 120based on signal branch measures obtained after de-scrambling and/orde-spreading of the received signal has been performed, the reliabilityof the method will be improved.

When an antenna branch 120 receives a signal branch 305 carryinginformation relating to several radio links, measurements could be madeon all of these links. The signal branch measure values couldadvantageously be collected at the traffic channel of active radiolinks, for example as specified in 3GPP TS 25.215, Physical layermeasurements: (FDD). Measurements could for example advantageously beperformed on the pilot bits of the Dedicated Physical Control Channel(DPCCH). The pilot bits include a well defined pattern of several bits,thus simplifying the measurement procedure. Alternatively, the signalbranch measure may be collected on the random access channel (RACH). Inparticular, the signal branch measure may advantageously be collected onthe pre-amble part of the random access channel.

Base Station Including a Rake Receiver

In one aspect of the invention in which the signal processing part 110of the radio base station 100 comprises a rake receiver, the signalbranch measures can be obtained from an output signal from the rakereceiver.

A rake receiver of a radio base station 100 typically operates in thebase band to compensate for the fact that a received signal has beentransmitted over a multi-path channel and has therefore been smeared dueto a difference in path length of the different paths. In the rakereceiver, signal components that represent the same data but haveexperienced different path lengths (and are therefore received by theantenna system 105 at different points in time) are synchronised.

A rake receiver of L rake fingers operates to periodically select the Lstrongest samples of a number of samples having been sampled from thereceived signal within a certain time period T_(rake), and thenprocesses these samples individually prior to adding them into an outputsignal (often by first multiplying them with a weight factor).

The concept of a rake receiver is schematically illustrated in FIG. 3. Asignal 300 is illustrated to comprise three different signal componentswhich have traveled along three different signal paths. The signal isreceived by a radio base station 100 comprising two antenna branches120A and 120B, so that a signal branch 305A of signal 300 is received bythe antenna branch 120A, and a signal branch 305B is received by theantenna branch 120A. For illustration purposes, the antenna branches 120of FIG. 3 are illustrated to be space diversity antenna branches,however, any antenna diversity method could be employed.

The radio base station 100 of FIG. 3 further comprises two samplers 310Aand 310B, connected to the antenna branches 120A and 120B, respectively.(The two samplers 310A and 310B could be implemented in the samephysical processor, or in different physical processors. Furthermore, asampler 310 could be implemented in one or more physical processors).

The signal branches 305A and 305 B are fed from the antenna branches120A and 120B to the samplers 310A and 310B, where, inter alia, thesamplers 310 operate to sample the signal branches 305 at regularintervals. The sampling is illustrated in FIG. 3 as a number of signalsamples 315, the exemplary signal samples 315 which have been sampled bysampler 310A in FIG. 1 being indicated as samples A1-A13, and the signalsamples 315 which have been sampled by sampler 310B being indicated assamples B1-B13. The height of a signal sample 315 corresponds to thesignal strength of the received signal branch 305.

A rake receiver 320 including L different rake fingers 325 is connectedto the samplers 310A and 310B of FIG. 3, where L=3 in the example ofFIG. 3. The rake receiver 320 operates to select the L strongest samplesduring a time interval T_(rake), and process these L samples in the Lrake fingers 325. In the example given by FIG. 3, the L (=3) strongestsamples sampled during the time interval T_(rake), (which in theillustration corresponds to 12 samples), are all derived from the signalbranch 305A received by antenna branch 120A, and these 3 strongestsamples, i.e. samples A1, A6 and A10, are processed in the 3 rakefingers 325 of the example.

A reason for a first antenna branch 120 to receive a weaker signalbranch 305 than the other signal branch(es) of a radio base station 100could be that the signal branch 305 received by the first antenna branch120 has experienced more fading than the other signal branches. However,another reason could be that there is something wrong with the firstantenna branch 120.

Number of Samples Selected by Rake Fingers Used as Signal Branch Measure

In one embodiment of the invention, the number of samples selected bythe rake receiver from a signal branch 305, or a measure derived fromthis number, is used as the signal branch measure of the signal branch305. In other words, the number of samples selected by the rake fingers320 from a particular antenna branch 120 is compared to the number ofsamples selected from the other antenna branch(es) 120 over a period oftime. If the rake receiver is arranged to select the L strongestsamples, regardless of which antenna branch 120 the samples originatefrom, a comparison between the different antenna branches 120 of anantenna system 105 could be based on the number of rake fingers 325 thatselect a sample from each of the signal branches within a time intervalT_(rake). By performing this comparison, an indication is given as towhether the antenna branches 120 operate equally well, or whether thereis an erroneous behaviour of one or more antenna branches 120. If therake receiver 320 only has one rake finger 325, the percentage ofsamples selected by the rake finger 325 from one signal branch 305 couldbe used as a signal branch measure.

SIR as a Signal Branch Measure

In a mobile communications system operating according to a CDMAstandard, the signal branch measure can advantageously be calculated asa measure of the signal-to-noise-and-interference-ratio (SIR) on thesignal after de-scrambling of the signal and/or after de-spreading ofthe signal has been performed. As discussed above, SIR of the signal ishigher after de-scrambling and de-spreading, and the probability ofdiscerning any difference in SIR between the different antenna branches120 increases as compared to performing the SIR calculation on thesignal prior to de-scrambling and/or de-spreading. Thus, lessmeasurement values are required than if the SIR-comparison were to bemade in the radio part of the signal processing part 110. This means,inter alia, that statistically accurate results can be obtained alsowhen the traffic is low. i.e. when there are few active mobile stationstransmitting to the radio base station 100.

Many implementations of a radio base station 100 employ a mechanism fordetermining a target value for the SIR value of a radio link. In suchimplementations, the target SIR value for different radio links need notbe the same, and the target SIR value for a radio link may vary withtime. Hence, to simply use the SIR measurement values as the signalbranch measure may not give a correct result, since the target SIR valuemay not have been the same at the different measurement occasions ordifferent radio links. In order to account for this, one could forexample ensure that only SIR measurement values that have the same SIRtarget value are included in the comparison. Alternatively, a relationbetween the SIR measurement value and the corresponding SIR targetvalue, referred to as SIR^(error), could be used as the signal branchmeasure. SIR^(error) could be expressed as:SIR^(error)=SIR^(measured)−SIR^(target)  (1).where SIR^(target) and SIR^(measured) have been measured/averaged overthe same period of time. SIR^(error) could alternatively be expressed asa ratio between SIR^(measured) and SIR^(target). In the following, whenthe use of SIR as a signal branch measure is discussed, this should beconstrued to include the use of SIR^(error). as well as a measure ofSIR, as a signal branch measure.

SIR can advantageously be used as a signal branch measure in a radiobase station 100 comprising a rake receiver 320, and the signal branchmeasure of a signal branch 305 can be calculated as the base bandsignal-to-interference-and-noise-ratio of the branch at the rakereceiver, SIR_(rake). In a radio base station 100 operating accordingthe WCDMA standard, the signal received by the rake receiver 320 hasnormally been de-scrambled and de-spreaded by the Application ofappropriate channelisation codes prior to arriving at the rake receiver320. SIR_(rake) can be expressed as:

$\begin{matrix}{{{SIR}_{rake} = {\sum\limits_{k = 1}^{m}\frac{S_{k}}{n_{k}}}},} & (2)\end{matrix}$where S_(k) and n_(k) are the signal strength and the estimatednoise-and-interference level, respectively, of the part of the signalbranch processed by the k^(th) rake finger 325 during a time interval T,and m is the number of rake fingers 325 that have selected samples 315from signal branch 305 during the time interval T (m≧1). Alternatively,a SIR measurement can be obtained at the output of the rake receiver320, where the outputs of the different rake fingers 325 has beencombined into one signal.

In a radio base station 100 operating according to the WCDMA standard, ameasurement of SIR can advantageously be obtained on the DedicatedPhysical Control Channel (DPCCH) by calculating the ratio between theReceived Signal Code Power (RSCP) and the Interference Signal Code Power(ISCP), and multiplying the ratio with the Spreading Factor (SP). TheRSCP is the unbiased measurement of the received power on one code, andthe ISCP is the interference on the received signal (see e.g. thetechnical specification 3GPP TS 25.215).

There may be reasons to why a SIR-value is occasionally not obtained foran antenna branch 120. Thus, in order to increase the comparability ofthe SIR-values of the different antenna branches 120, one could ensurethat for a SIR-measurement of a first signal branch 305 included in thecomparison of step 215 of FIG. 2, there exists a correspondingSIR-measurement of the signal branch 305B to which the first signalbranch 305 is to be compared. Furthermore, any SIR-measurements whichhave been collected when the transmitting mobile station was notsynchronised with the radio base station 100 can advantageously beexcluded from the comparison of step 215.

As mentioned above, the signal branch measure values could be collectedat the traffic channels, or on the random access channels (RACH). Thesignal processing part 110 of the radio base station 100 typicallymeasures the power of the signal branches 305 of the preambles of theRACH at regular intervals (e.g. even half chip), and/or the power of thecombination of the different signal branches 305. Interference & noiseestimates of the pre-ambles can also be obtained. An appropriate signalbranch measure could then be the SIR_(rake) of the signal branch 305 asgiven in equation (2). An advantage of performing the signal branchmeasure measurements on the random access channels is that the randomaccess channels are often processed on the same circuit board for allantenna branches 120, and the process of identifying any erroneousantenna branch 120 could easily be centralised. To improve the erroridentification process, one could preferably choose to base the signalbranch measure measurements only on the random access preambles thatresult in an acknowledgment (typically in Layer 1), since theacknowledgement indicates that the signal quality lies above a certainthreshold.

Comparison Between Different Antenna Branches

According to the invention, a number of signal branch measure values arecollected for at least two antenna branches 120, and the signal branchmeasure values of the different antenna branches 120 are compared inorder to detect any erroneous behaviour of any of the antenna branches120. The comparison between the signal branch measure values of thedifferent antenna branches can be made in different ways.

When a measurement of SIR is used as a signal branch measure, either asmeasured on traffic channels or on the random access channel, a simpletest of the appropriate function of the different antenna branches 120could be a check as to whether a SIR-value has been obtained for allantenna branches 120—if a branch 120 does not generate any SIR-valuesover a longer period of time, this may be an indication of malfunctionof this antenna branch 120.

The comparison between the signal branch measures performed in step 215of FIG. 2 could for example be performed by comparing the average signalbranch measures of different signal branches (where the average valuesmay for example be obtained by measuring the signal branch measures overa pre-determined period of time, or by including a predetermined numberof signal branch measure values in the average signal branch measurecalculation). Over a period of time, the ratio between the averagesignal branch measures from two different antenna branches 120 shouldapproach 1, whereas the difference should approach 0, when both antennabranches 120 operate equally well. If the comparison performed in step215 involves calculating the difference between the average value of thesignal branch measure of two antenna branches 120, a predeterminedthreshold value could be used in the check for erroneous behaviourperformed in step 220 of FIG. 2, so that when the magnitude of thisdifference exceeds the threshold value, step 225 should be entered andaction should be taken. Depending on the signal branch measure used,different threshold values could be applied. When the comparison in step215 involves calculating a ratio between signal branch measures of twodifferent signal branches, the check of step 220 could include a checkas to whether the ratio R lies within an interval [R_(min), R_(max)],where R_(min)<1<R_(max), and where the values of R_(min) and R_(max) arechosen in accordance with which signal branch measure is used.

FIG. 4 is a flowchart illustrating an embodiment of the inventive methodwherein the comparison of step 215 is performed as a comparison betweenaverage values of the signal branch measures of two different antennabranches 120, and the number of samples 315 selected by the rakereceiver 320 of a radio base station 100 from a signal branch 305 duringa time period T is used as a signal branch measure of the signal branch305.

In the example of FIG. 4, a radio signal 300 is received in differentantenna branches 120 of the radio base station 100 as different signalbranches 305. In step 405, the number samples 315 from a signal branch305 that are selected by a rake finger 325 is counted for the differentsignal branches 120 during a time period T, which could for example bethe time interval T_(rake). In step 407 it is checked whether the numberof time periods during which the selected number of samples have beencounted are sufficient. If not, step 405 is re-entered, and if so, step410 is entered. In step 410, the average number of samples selected froma signal branch 305 is calculated for the different signal branches 305.In step 415, the average number of samples of the different signalbranches 305 are compared with each other. In step 420, it is checkedwhether the comparison result indicates any erroneous behaviour of anyof the antenna branches 120.

The comparison between the number of samples selected from the twodifferent antenna branches 120 performed in step 415 could e.g. be madeas a ratio of the number of samples selected from the two antennabranches 120, or as a difference. If the comparison is performed as aratio, the step of calculating an average number of selected samplescould advantageously be omitted, and instead, the time period T in step405 can be set to an appropriate length, typically in the order ofminutes (e.g. 2 minutes). Typical values of the end values R_(min) andR_(max) of the interval within which the ratio R should lie could thene.g. be [0.5:2].

The average value of a signal branch measure (SBM). SBM, canadvantageously be calculated by adding signal branch measure values fromseveral or all radio links received by an antenna branch 120, and byadding the signal branch measure values obtained during a time period:

$\begin{matrix}{\overset{\_}{SBM} = {\frac{1}{M}{\sum\limits_{l = 1}^{M}{{SBM}^{\; l}.}}}} & (3)\end{matrix}$where M is the total number of measurement values of the signal branchmeasure collected on a signal branch 305 during the time period. Asmentioned above, the total number of measurement values. M, can includemeasurement values from different radio links. If the total number ofmeasurement values is the same for each antenna branch 120 to beincluded in the comparison, an aggregate signal branch measure could becalculated instead of an average signal branch measure value of eq. (3),i.e. the division by M could be omitted from eq. (3).

When a signal-to-noise-and-interference measure is used as a signalbranch measure, the comparison made in step 215 of FIG. 2 couldadvantageously be performed as a calculation of the difference betweenthe logarithm of the average value of SIR, SIR of a first antenna branch120A and the logarithm of SIR of a second antenna branch 120B. Anexemplar) expression for obtaining a comparison result between the twobranches 120A and 120B is provided by equation (4):SIR^(A-B) =10 log₁₀ SIR^(A) −10 log₁₀ SIR^(B)   (4)

The magnitude of SIR^(A-B) of equation (4) is a measure of how similarthe signal branches 305, received by the two antenna branches 120A and120B, are. If the magnitude of SIR^(A-B) is below a certain threshold,then both antenna branches 120A and 120B would be considered to operatenormally. If not, depending of the sign of SIR^(A-B) , an error would besuspected in either signal branch 120A or in signal branch 120B.

In alternative embodiment of the comparison step 215 of FIG. 2, thecomparison between different signal branches could be performed on a permeasurement basis rather than on an average signal branch measurementbasis. The result of such per-measurement-comparison will in thefollowing be referred to as the per-measurement-comparison-result. Theper-measurement-comparison-result could be obtained by calculating theratio or difference between two signal branch measure values, obtainedat the same time, on two different antenna branches 120 (needless tosay, a measurement value of a branch measure does not necessarilyreflect an instantaneous value of the branch measure, but could be anaverage value of the branch measure over a period of time). By analysinga set of such per-measurement-comparison results, an indication ofwhether the operation of any of the antenna branches 120 is erroneouscan be obtained. The analysis could e.g. include calculation of theaverage value of the per-measurement-comparison results:

$\begin{matrix}{{\overset{\_}{{SBM}^{diff}} = {\frac{1}{M}{\sum\limits_{l = 1}^{M}\left( {{SBM}_{A}^{l} - {SBM}_{B}^{l}} \right)}}},} & (5)\end{matrix}$where M is the number of per-measurement-comparison results. If themagnitude of SBM^(diff) exceeds a threshold value, this indicatespossible malfunction of one of the antenna branches 120A or 120B. Thesign of SBM^(diff) indicates which of the antenna branches 120experiences the problems. When using SIR as a signal branch measure(SBM), an advantage of calculating SBM^(diff) instead of calculating thedifference between the average values of the signal branch measuresaccording to eq. (3) is that any differences in target values of thesignal branch measures included in the averaging will be compensatedfor. The average value of the per-measurement-comparison results canalternatively be expressed as

$\begin{matrix}{\overset{\_}{{SBM}_{A - B}} = {10\mspace{11mu}\log\;\frac{1}{M}{\sum\limits_{l = 1}^{M}{\left( {{{SBM}_{A}^{l} - {SBM}_{B}^{l}}} \right).}}}} & (6)\end{matrix}$

This expression can be particularly useful when using SIR as a signalbranch measure, since appropriate resolution can be obtained. However,expression (6) does not give any indication of which of the two antennabranches 120 experiences problems. Further analysis of the set ofper-measurement-comparison results would be required in order to obtainsuch information.

The distribution of the per-measurement-comparison results often providefurther information regarding any possible erroneous behaviour of any ofthe antenna branches 120. By analysing the distribution of suchper-measurement-comparison-results, for example by plotting thenumber-of-measurement-results vs. the per-measurement-comparison-resultsin a distribution graph, information regarding the status of the antennabranches 120 can be obtained. If the antenna branches 120 operateideally, the distribution of the difference between the measurementresults should be centred around zero (the ratio should be centredaround 1). The standard deviation of the distribution can giveadditional information. For example, if the feeder of a first antennabranch 120 is connected to signalling part 110 serving a first cell, andthe feeder of a second antenna branch 120 is connected to a signallingpart 110 serving a second cell, the standard deviation of thedistribution will be large. Problems with the antenna equipment orlosses in the radio frequency path would give a ratio of the standarddeviation and the absolute value of the difference that is smaller thanwould be obtained if there is an antenna diagram mismatch, etc.

An illustration of a per-measurement distribution graph is provided inFIG. 5, in an embodiment in which SIR_(rake) is used as the signalbranch measure. The difference in SIR_(rake) has been calculated for twodifferent antenna branches A and B for a number of measurements, and thedistribution of SIR_(rake) ^(A)−SIR_(rake) ^(B) for these measurementsis plotted in the graph of FIG. 5. The distribution illustrated in FIG.5 indicates that the antenna branch A is functioning less well thanantenna branch B, since the average value is less than zero.Alternatively, the comparison could be a visual comparison, where theper-measurement-values are plotted in the same graph for the antennabranches 120 to be compared, rather than the difference or ratio betweenthe signal measures of the different antenna branches 120.

Table 1 illustrates examples of threshold values of the distributionthat could be used in notifying an operator of the antenna system 105 ofa possible error. Threshold values are given for the magnitude of theaverage value of the distribution, the standard deviation of thedistribution as well as the ratio of the standard deviation and theabsolute value of the average. Table 1 relates to the situation wherethe SIR_(rake) is used as the signal branch measure, and the comparisonbetween two different antenna branches is performed as a difference (cf.FIG. 5). The actual thresholds given in Table 1 are not to be seen asabsolute numbers, but as a guidance for which thresholds could be usedin an implementation of the invention.

TABLE 1 Threshold values for indication of errors in the antenna system105. Error Std |average| $\frac{std}{|{average}|}$ Swapped feeder std >20 dB Losses in RF path >3 dB <3 dB Antenna diagram mismatch ≦3 dB >3 dBNone of the above ≦20 dB ≦3 dB

The above described evaluation of an antenna system 105 couldadvantageously be implemented so that the evaluation result, either inthe form of all or some of the collected signal branch measure values,and/or in the form of error indication(s), are transmitted over acontrol channel to an operations & maintenance centre. A mobile radiocommunications system comprises a large number of radio base stationswhich are distributed over a vast geographical area, and the possibilityof efficient remote monitoring of the antenna systems 105 of the systemwill greatly improve the efficiency of the operations & maintenance ofthe mobile radio system.

FIG. 6 is a schematic illustration of an exemplary signal processingpart 110 of a radio base station 100, operating according to spreadspectrum multiple access/code division multiple access, in which anembodiment of the invention has been implemented. The exemplary signalprocessing part 110 comprises a radio part 605, a de-scrambler 607, ade-spreader 610, a rake receiver 320 and a signal branch comparator 615.The signal processing part 110 has an input connected to the feeder 115,on which signal branches 305 are received from the antenna system 105.The signal processing part 110 converts the signal branches 305 intodigital signal branches, which are de-scrambled in de-scrambler 605,de-spread in de-spreader 610 and processed by the rake receiver 320 asdescribed in relation to FIG. 3. The output 620 from the rake receiver325 delivers an output signal, where the contribution from the differentsignal branches 305 has been taken into account in the manner describedin relation to FIG. 3. An input 623 of the signal branch comparator 615is connected to an output 625 of the rake receiver 320 in a manner sothat the signal branch comparator 615 receives data indicative of thesignal branch measures of the different signal branches 305.

Depending on the implementation of the rake receiver 325 and the signalbranch comparator 615, outputs 620 and 625 could be implemented as thesame output, or different outputs. The signal branch comparator 615comprises software and hardware for performing an analysis of thereceived data according to any of the embodiments described above.Furthermore, the signal branch comparator 615 has an output 630 foroutputting the result of the performed analysis.

When no de-scrambling or de-spreading of the signal is required, thede-scrambler 607 or the de-spreader 610, respectively, could be omitted.The input 623 of the signal branch comparator 615 need not be connectedan output 625 of the rake receiver 320. In an embodiment of theinvention where the signal branch measure is not obtained from an outputfrom the rake receiver 320 (e.g. when the radio base station 100 doesnot comprise a rake receiver 320), the input 623 of the signal branchcomparator 615 is connected to receive a signal indicative of therespective signal branch measures of the different antenna branches 120.

In implementations where the evaluation of the antenna system 105 can beperformed on a on-demand basis, the comparator 615, or anotherappropriate entity of signal processing part 110, could comprise aninput for receiving instructions to perform an evaluation of the antennasystem 105.

In the embodiment illustrated in FIG. 6, the signal branch comparator615 is implemented in the radio base station 100. However, the signalbranch comparator 615 could alternatively be implemented in a monitoringsystem separate from the radio base station 100.

In the above, the invention has been described in terms of the antennasystem 105 of a radio base station 100. However, the invention isequally applicable to any digital radio apparatus employing antennadiversity. Furthermore, although the invention has been described inrelation to receiving antennas, it could also be used for monitoring thestatus of transmitting antennas by utilising the inherent receivingproperties that most transmitting antennas have. The invention may alsobe used to check the status of an antenna system 105 where differentantenna elements are used for receiving signals on different channelsfrom transmitters within the same coverage area. In such a situation,the different antenna branches 120 are not for creating antennadiversity, but for increasing the number of channels that can be activeat the same time. By making an antenna branch 120 eavesdropping on theother antenna branch(es) 120 covering the same area, an evaluationaccording to the invention would indicate whether any errors in any ofthe antennas exists, for covering the same area on different channelsreceiving.

In order to increase the reliability of the inventive evaluationprocess, a combination of the different embodiments described abovecould be employed. For example, an analysis of the performance ofantenna branches 120 could be based on the use of two different signalbranch measures, or more. Furthermore, the comparison could include botha comparison of the average signal branch measure values, and ananalysis based on per-measurement-comparison-results.

The invention claimed is:
 1. A method of monitoring an antenna systemconnected to a radio device comprising a rake receiver, the antennasystem comprising at least two antenna branches, the method comprising:receiving radio signals as a first signal branch in a first antennabranch; receiving the radio signals as a second signal branch in asecond antenna branch; obtaining first and second streams of samplesfrom the first and second signal branches, and selecting strongest onesof the samples for processing via the rake receiver, without regard towhich antenna branch the selected samples originate from; anddetermining performance differences between the first and second antennabranches based on the respective numbers of samples selected from thefirst and second signal branches, or based on respectivesignal-to-noise-and-interference ratios of the first and second signalbranches, in view of a signal-to-noise-and-interference ratio target foran associated radio link.
 2. The method of claim 1, wherein selectingthe strongest samples for processing via the rake receiver comprises,for each time interval in a plurality of successive time intervals,selecting the strongest ones of the samples obtained from the first andsecond signal branches.
 3. The method of claim 2, wherein determiningperformance differences between the first and second antenna branchesbased on the respective numbers of samples selected from the first andsecond signal branches comprises comparing the respective numbers ofsamples selected from the first and second signal branches over theplurality of successive time intervals, or comparing a measure derivedfrom the respective numbers.
 4. The method of claim 1, whereindetermining performance differences between the first and second antennabranches based on the respective numbers of samples selected from thefirst and second signal branches comprises, for one or more periods oftime, comparing the number of samples selected from the first stream ofsamples for processing via the rake receiver to the number of samplesselected from the second stream of samples for processing via the rakereceiver.
 5. The method of claim 1, wherein determining performancedifferences between the first and second antenna branches based on therespective numbers of samples selected from the first and second signalbranches comprises, for one or more periods of time, comparing thenumber of samples selected from the first signal branch to the number ofsamples selected from the second signal branch.
 6. The method of claim1, further comprising determining that one of the first and secondsignal branches is faulty responsive to the performance differencesdetermination indicating that one of the first and second signalbranches is generally weaker than the other one.
 7. The method of claim1, further comprising initiating an alarm signal or a furtherinvestigative process in the radio device, responsive to the performancedifferences determination indicating that one of the first and secondsignal branches is generally weaker than the other one.
 8. The method ofclaim 1, wherein the radio signals are received on a random accesschannel, such that the first and second streams of samples are obtainedfrom radio signals received on the random access channel.
 9. The methodof claim 1, wherein determining performance differences between thefirst and second antenna branches based on the respectivesignal-to-noise-and-interference ratios of the first and second signalbranches comprises determining signal-to-noise-and-interference ratioerrors for the first and second signal branches, respectively, as thedifference between measured signal-to-noise-and-interference ratiodetermined for the first and second signal branches, and a targetsignal-to-noise-and-interference ratio.
 10. The method of claim 1,further comprising measuring the respectivesignal-to-noise-and-interference ratios of the first and second signalbranches using descrambled and despread signal samples.
 11. A radioreceiver including a radio device comprising a rake receiver coupled toan antenna system comprising at least two antenna branches, the radioreceiver configured to: receive radio signals as a first signal branchin a first antenna branch; receive the radio signals as a second signalbranch in a second antenna branch; obtain first and second streams ofsamples from the first and second signal branches, and selectingstrongest ones of the samples for processing via the rake receiver,without regard to which antenna branch the selected samples originatefrom; and determine performance differences between the first and secondantenna branches based on the respective numbers of samples selectedfrom the first and second signal branches, or based on respectivesignal-to-noise-and-interference ratios of the first and second signalbranches, in view of a signal-to-noise-and-interference ratio target foran associated radio link.
 12. The radio receiver of claim 11, whereinthe radio receiver is configured to select the strongest samples forprocessing via the rake receiver by, for each time interval in aplurality of successive time intervals, selecting the strongest ones ofthe samples obtained from the first and second signal branches.
 13. Theradio receiver of claim 12, wherein the radio receiver is configured todetermine performance differences between the first and second antennabranches based on the respective numbers of samples selected from thefirst and second signal branches by comparing the respective numbers ofsamples selected from the first and second signal branches over theplurality of successive time intervals, or comparing a measure derivedfrom the respective numbers.
 14. The radio receiver of claim 11, whereinthe radio receiver is configured to determine performance differencesbetween the first and second antenna branches based on the respectivenumbers of samples selected from the first and second signal branchesby, for one or more periods of time, comparing the number of samplesselected from the first stream of samples for processing via the rakereceiver to the number of samples selected from the second stream ofsamples for processing via the rake receiver.
 15. The radio receiver ofclaim 11, wherein the radio receiver is configured to determineperformance differences between the first and second antenna branchesbased on the respective numbers of samples selected from the first andsecond signal branches by, for one or more periods of time, comparingthe number of samples selected from the first signal branch to thenumber of samples selected from the second signal branch.
 16. The radioreceiver of claim 11, wherein the radio receiver is configured todetermine that one of the first and second signal branches is faultyresponsive to the performance differences determination indicating thatone of the first and second signal branches is generally weaker than theother one.
 17. The radio receiver of claim 11, wherein the radioreceiver is configured to initiate an alarm signal or a furtherinvestigative process in the radio device, responsive to the performancedifferences determination indicating that one of the first and secondsignal branches is generally weaker than the other one.
 18. The radioreceiver of claim 11, wherein the radio receiver is configured toreceive radio signals on a random access channel, and wherein the radiosignals received on the first and second antenna branches are receivedon the random access channel, such that the first and second streams ofsamples are obtained from the radio signals received on the randomaccess channel.
 19. The radio receiver of claim 11, wherein the radioreceiver is configured to determine performance differences between thefirst and second antenna branches based on the respectivesignal-to-noise-and-interference ratios of the first and second signalbranches by determining signal-to-noise-and-interference ratio errorsfor the first and second signal branches, respectively, as thedifference between measured signal-to-noise-and-interference ratiodetermined for the first and second signal branches, and a targetsignal-to-noise-and-interference ratio.
 20. The radio receiver of claim11, wherein the radio receiver is configured to measure the respectivesignal-to-noise-and-interference ratios of the first and second signalbranches using descrambled and despread signal samples.